1. Field of the Invention
The present invention relates to heat resisting steels used for such large forged products as a rotor or a disk of a steam or gas turbines. More particularly, the invention relates to forging heat resisting steels for forming a gas turbine rotor or a disk having a creep rupture strength within a high temperature range of 400 to 600.degree. C. and excellent tenacity at room temperature.
2. Description of the Related Art
In recent years, we have seen a tremendous increase in attention paid to a gas turbine as an efficient and clean power generation system including a combined cycle power generation plant composed by combing a gas turbine and a steam turbine. For further improvement in the efficiency of such a gas turbine or a combined cycle power generation plant, the gas temperature at the entrance of the gas turbine has been increased. Currently, 1300.degree. C. class plants have been put to practical use, and 1500.degree. C. class plants are under development.
With such increase in the gas temperature of the entrance, better high-temperature strength and tenacity have also been demanded for moving and stationary blades, a combustor, a gas turbine, a compressor disk, and so on. As a material for a disk which is largest among rotary parts of the gas turbine, an Ni base alloy such as Inconel has started to be used with higher temperatures. However, out of consideration for the difficulty of manufacturing large parts made of heat resisting materials and increases in costs associated with increasing output requirements, stronger demand for improved performance of ferritic materials such as 12% Cr steels or low alloy steels have currently been made.
As materials for the turbine disk of a gas turbine or a jet engine, alloy steel forged products each having chemical composition shown in Tables 1 to 4 have conventionally been used. Table 1 shows composition examples of 3.5Ni--Cr--Mo--V steel (low alloy steel disk material). Table 2 shows composition examples of Cr--Mo--V steel (low alloy steel disk material). Table 3 shows composition examples of 12% Cr steel disk material. Table 4 shows composition examples of a Fe base heat resisting alloy disk material.
TABLE 1 __________________________________________________________________________ COMPOSITION EXAMPLE OF LOW ALLOY STEEL DISK MATERIAL (3.5Ni--Cr--Mo--V STEEL) STAN- COMPOSITION (WT. %) DARD C Mn P S Si Ni Cr Mo V __________________________________________________________________________ ASTM.A471 (NOTE) MAX. MAX. MAX. 0.15- 2.00- 0.75- 0.20- MINI. CIass 1-9 0.70 0.015 0.015 0.35 4.00 2.00 0.70 0.05 __________________________________________________________________________ (NOTE): CLASS 4 AND 5; UP TO 0.35%, CLASS 6,7,8 AND 9; UP TO 0.40%, OTHER CLASS; UP TO 0.28%
A 3.5Ni--Cr--Mo--V steel (low-alloy steel) disk material containing several percent of Ni like that shown in Table 1 has a relatively high 0.2% yield strength (0.2% yield strength is simply referred to as yield strength, hereinafter) which is 70 to 120 kg/mm.sup.2 and high tenacity in which V-notch Charpy impact absorbing energy is 5 to 10 kg-m or higher at 25.degree. C. In addition, dissolving, forging and heat treatment are relatively easy, costs are low and the material is easily available. However, if use temperature (metal temperature of a disk material) exceeds 300 to 350.degree. C., this disk material enters a creep region and, for material strength designing, creep must be taken into consideration. Consequently, designing becomes complex and a strength such as tensile strength or yield strength shows a softening phenomenon in which strength is reduced more as the use time becomes longer. Further, if this disk material is used for several hundreds to several tens of thousands of hours within the temperature range of 350 to 500.degree. C., its tenacity is substantially reduced because of temper brittleness. Such a disadvantage is inherent in low-alloy steels containing Ni of several percent or more, which is used for a disk material having improved strength and tenacity by performing quenching and tempering for thermal refining.
TABLE 2 __________________________________________________________________________ COMPOSITION EXAMPLE OF LOW ALLOY STEEL DISK MATERIAL (Cr--Mo--V STEEL) STAN- COMPOSITION (WT. %) DARD C Mn P S Si Ni Cr Mo V __________________________________________________________________________ ASTM.A471 0.27- 0.70- MAX. MAX. MINI. MAX. 0.85- 1.00- 0.20- Class 10 0.37 1.00 0.015 0.015 0.20 0.50 1.25 1.50 0.30 __________________________________________________________________________
For Cr--Mo--V steels (low-alloy steels) shown in Table 2, costs are low and a material made of these steels is easily available as in the case of 3.5Ni--Cr--Mo--V steels described above with reference to Table 1. Different from 3.5Ni--Cr--Mo--V steels, however, these steels do not show any substantial softening phenomenon or temper brittleness. Since the Cr--Mo--V steels do not enter a creep region until use temperature reaches 430 to 480.degree. C., the use temperature can be increased by 100 to 200.degree. C. more than that for 3.5Ni--Cr--Mo--V steels. However, tenacity is not so high as that for 3.5Ni--Cr--Mo--V steels. Especially, if tensile strength or yield strength is to be increased, tenacity will be substantially reduced. For example, if temper refining is performed so as to set yield strength to a high level of 70 to 80 kg/mm.sup.2 or more, V-notch Charpy impact energy will be greatly reduced to 1 to 2 kg-m or lower at a temperature of 25.degree. C. Further, the material made of these steels enters a creep region even at a temperature of about 400.degree. C. and, in this temperature region, notch weakening occurs (a notch creep rupture strength becomes weaker than a smooth creep rupture strength). Consequently, if Cr--Mo--V steels are used for a disk material, temper refining (quenching or tempering) for greatly increasing the strength cannot be performed and, typically, a strength level is set lower than that for 3.5Ni--Cr--Mo--V steels. Usually, yield strength at room temperature is 60 to 70 kg/mm.sup.2 or lower.
TABLE 3 ______________________________________ COMPOSITION EXAMPLE OF 12% Cr STEEL DISK MATERIAL PRO- DUCT COMPOSITION (WT. %) NAME C Si Mn Nb Cr MO Ni W V ______________________________________ Jessop - 0.16 0.3 0.7 0.25 11.6 0.6 -- -- 0.30 Saville H46 (NOTE 1) Jethete 0.10 0.3 0.7 -- 12.0 1.8 2.4 0.35 -- M152 (NOTE 2) Firth - 0.13 0.5 1.0 0.8 11.2 0.6 -- -- 0.30 Vickers 448 (NOTE 3) ______________________________________ (NOTE 1): PRODUCT BY U.S JessopSaville Ltd. (NOTE 2): PRODUCT BY U.S Samuel Fox & Co. Ltd. (NOTE 3): PRODUCT BY U.S FirthVickers Stainless Steel Ltd.
For 12% Cr steels shown in Table 3, high strength and high tenacity are secured by making an alloy contain, in addition to Cr of about 12%, Ni, Mo and V. These 12% Cr steels have higher corrosion resistance and higher oxidation resistance compared with the foregoing 3.5Ni--Cr--Mo--V steels and the Cr--Mo--V steels both as low alloy steels. Strength at high temperatures and tenacity at room temperature are relatively better compared with the above-mentioned Cr--Mo--V steels. However, any great increase in strength or tenacity cannot be expected.
TABLE 4 __________________________________________________________________________ COMPOSITION EXAMPLE OF Fe BASE HEAT RESiSTING ALLOY DISK MATERIAL STAN- COMPOSITION (WT. %) DARD C Mn Si P S Cr Ni MO Ti Cu Al V B __________________________________________________________________________ ASTM A638 Grade MAX MAX MAX MAX MAX 12.00 21.00- 2.50- 1.55- MAX MAX -- 0.0010- 662 0.08 1.50 1.0 0.040 0.030 15.00 28.00 3.50 2.00 0.50 0.35 0.010 Grade MAX MAX MAX MAX MAX 13.50- 24.00- 1.00- 1.90- -- MAX 0.10- 0.0010- 660 0.08 2.00 1.00 0.040 0.030 16.00 27.00 1.50 2.35 0.35 0.50 0.010 __________________________________________________________________________
A Fe base heat resisting alloy like that shown in Table 4 contains large quantities of Ni and Cr. Accordingly, since a material made of this alloy has high corrosion resistance and high oxidation resistance and enters a creep region in a temperature of 500 to 580.degree. C. or higher, high temperature strength and uniform austenitic structure can be provided. No brittle fracture occurs and thus brittleness needs not be taken into consideration. In addition, since its strength is provided by precipitation hardening of a .gamma.' phase Ni.sub.3 (Al.cndot.Ti)! intermetallic compound during solution heat treatment or aging treatment, different from the case of the foregoing low-alloy steels (tempered bainite structure) or 12% Cr steels (tempered martensite structure), a mass effect needs not be taken into consideration during temper refining. However, as this alloy contains large quantities of expensive alloy elements such as Ni, Cr or Mo and highly active alloy elements such as Al or Ti, a normal dissolving method cannot be employed. A higher level dissolving technique such as vacuum high frequency dissolving or vacuum arc melting is necessary. Consequently, costs are very high, which may be 5 to 10 times as high as those for the usual low-alloy steels. Furthermore, because of the recent attainment of large capacity for a turbine, the unit weight of a necessary disk is 6 to 8 tons or more. For manufacturing such large Fe-base heat resisting disk forged products, special facilities must be installed. At present, in Japan as well as abroad, the number of makers having such facilities is quite limited, amounting only to a few companies.
On the other hand, in recent years, efforts have been made to attain high efficiency and large capacity for a gas turbine. The entrance gas temperature for the gas turbine has to be raised for achieving improved thermal efficiency. Accordingly, a disk metal temperature is increased to 450.degree. C. or higher. It is difficult to limit this temperature increase to 300 to 350.degree. C. which is a highest use temperature range for the foregoing 3.5Ni--Cr--Mo--V steels. For the attainment of large capacity, the disk size is also increased. Thus, the unit weight of a disk forged product is very heavy, reaching 6 to 8 tons or higher.
In addition, for high temperature parts of the gas turbine such as a rotor or a disk, excellent material characteristics including high temperature strength and high tenacity must be provided. It is also necessary to limit changes in the material characteristics to a minimum at high use temperatures in a plant over long periods of time. Among the rotor or disk materials thus far used, especially 12% Cr heat resisting steels have been found to have good material characteristics including high-temperature strength and high tenacity. However, brittleness occurs following the long-time use of the conventional 12% Cr heat resisting steels. Thus, in order to deal with the gas temperature increase at the entrance, such brittleness must be suppressed. The brittleness of 12% Cr heat resisting steels is composed of reversible brittleness and non-reversible brittleness. The reversible brittleness can be overcome by rapidly cooling the material after heating the same close to a tempering temperature. The non-reversible brittleness cannot be overcome by such re-heating and rapid cooling. In the case of the former reversible brittleness, the contribution of temper brittleness caused by intergranular segregation of impurity elements may be large. For reducing this brittleness, the reduction of impurity elements or the reduction of Si or Mn is known to be effective. For low-alloy steels having large brittleness sensitivity such as 3.5Ni--Cr--Mo--V steels, the attainment of high purity is known to be very effective for reducing such brittleness sensitivity.